1. Field of the Invention
The present invention relates to a duobinary optical transmission apparatus using a duobinary optical transmission technique, and more particularly to a duobinary optical transmission apparatus using a semiconductor optical amplifier (SOA).
2. Description of the Related Art
In general, a dense wavelength division multiplexing (DWDM) optical transmission system transmits an optical signal using a single optical fiber in such a way that it enhances transmission efficiency. The optical signal comprises of a plurality of channels having different wavelengths. Also, the DWDM optical transmission system is widely used for superhigh-speed Internet networks. Such systems have a rapidly increasing data transfer quantity because it transmits optical signals irrespective of the transfer rate. Recently, systems for transmitting more than 100 channels via a single optical fiber using DWDM optical transmission methods have been commercially produced. In addition, other systems that simultaneously transmit more than 200 channels (each channel having a transfer rate of 40 Gbps) to accomplish a transfer rate of more than 10 Tbps are under development.
Such systems accommodate rapidly increasing data traffic as well as transfer requests for high-speed data transmissions of more than 40 Gbps. However, conventional optical intensity modulation methods using an non-return to zero (NRZ) method has a number of limitations. For example, one limitation is in increasing the transfer quantity because an abrupt interference and distortion between channels occurs in a prescribed zone less than a channel interval of 50 GHz. Further, nonlinear characteristics and dispersion are caused by propagating DC frequency components of a conventional binary NRZ transmission signal and high-frequency components spreading in the modulation procedure. Thus, the transmission distance is limited at high-speed transfer rates over 10 Gbps.
Recently, an optical duobinary technique has been intensively researched for obviating the transmission distance limitation caused by chromatic dispersion. The optical duobinary technique reduces the width of the transmission spectrum more than a general binary transmission method. The transmission distance in a dispersion limitation system is inversely proportional to a square of the transmission spectrum bandwidth. For example, when the transmission spectrum bandwidth is reduced by half, the transmission distance increases by four times. Also, a carrier wave frequency is suppressed in a duobinary transmission spectrum. This suppression reduces the limitations in output optical power caused by Brillouin Scattering stimulated in an optical fiber.
FIG. 1 is a block diagram of a conventional duobinary optical transmission apparatus. FIGS. 2a-2c are illustrate eye-diagrams of output signals at A, B and C nodes shown in FIG. 1, respectively.
Referring to FIG. 1, a conventional duobinary optical transmission apparatus includes a pulse pattern generator (PPG) 10 for generating a two-level NRZ electric pulse signal, a precoder 20 for encoding the two-level NRZ electric signal, low pass filters (LPFs) 30 and 31 for converting the two-level NRZ electric signal received from the precoder 20 into a three-level electric signal, and reducing the bandwidth of the three-level electric signal, drive amplifiers 40 and 41 for amplifying the three-level NRZ electric signal, and generating an optical modulator driving signal, a laser source 50 for generating a carrier wave; and a Mach-Zehnder-interferometer-type optical intensity modulator (MZ MOD) 60.
The two-level electric pulse signals generated by the PPG 10 are encoded by the precoder 20. An output eye-diagram at a node A is shown in FIG. 2a. 
The two-level binary signal generated from the precoder 20 is transmitted to the LPFs 30 and 31. The LPFs 30 and 31 each have a bandwidth corresponding to about ¼ of a clock frequency of the two-level binary signal. This excessive bandwidth restriction causes interference between codes, and the two-level binary signal is converted into a three-level duobinary signal resulting from this interference between codes. An output eye-diagram at a node B is shown in FIG. 2b. 
The three-level duobinary signal is amplified at the drive amplifiers 40 and 31, and the amplified three-level duobinary signal is used as a driving signal of the Mach-Zehnder-interferometer-type optical intensity modulator (MZ MOD) 60. The carrier wave generated from the laser source 50 modulates its own phase and its own optical intensity according to the driving signal of the MZ MOD 60. A two-level optical duobinary signal is thus generated. An output eye-diagram at a node C is shown in FIG. 2C.
Referring to FIG. 1, a reference character Q indicates an inverted signal of a signal Q. The signals Q and Q are respectively transmitted to a positive electrode (+) and a negative electrode (−) of the MZ MOD 60 with a dual electrode structure through the LPFs 30 and 31 and the drive amplifiers 40 and 41.
In this way, the MZ MOD 60 is classified into a Z-cut type MZ MOD and an X-cut type MZ MOD according to its own electrode structure. Referring to FIG. 1, the Z-cut type MZ MOD has a dual electrode and connects both of its own arms to the drive amplifiers 40 and 41 and the LPFs 30 and 31. This connection enables a three-level electric signal to be applied to each of the electrodes of the Z-cut type MZ MOD. Although the X-cut type MZ MOD, which has a single electrode, is not shown in the drawings, it connects one of its own arms to a drive amplifier and a LPF in order to transmit a three-level signal to one electrode.
However, such conventional duobinary transmission apparatuses are greatly affected by a pseudo random bit sequence (PRBS) because they generate a three-level electric signal using a LPF. Deterioration in the transmission characteristics increases with longer lengths of a PRBS. Thus, the overall system suffers. In particular, the system margin is greatly reduced for a 231-1 PRBS than a 27-1 PRBS. Typically, the slope along a signal's level change from a 0-level to a 1-level is different from the slope along a signal's level change from the 1-level to the 0-level. However, a duobinary optical transmission apparatus using LPFs suffers from increased jitters of the output waves since portions having different slopes are mutually summed, such that a first signal transition from 0-level to 1-level and a second signal transition from 1-level to 0-level are performed. This jitter problem is generated in both in the conventional Z-cut type or X-cut type structure. The dependency of such signal patterns limits the optical transmission operation.
In order to obviate the aforementioned problem, a structure without an electric LPF has been proposed. FIG. 3 is another conventional duobinary optical transmission apparatus using both a phase modulator and an optical band pass filter (BPF). Referring to FIG. 3, the PPG 10, a precoder 20, a modulator drive amplifier 40, and a laser source 50 are similar to those of FIG. 1. Such a duobinary optical transmission apparatus generates signals similar to duobinary optical output signals of FIG. 1 using a phase modulator 70 and an optical BPF 80 instead of an electric LPF.
Although, the aforementioned duobinary optical transmission apparatus guarantees a predetermined transmission quality according to the length of a PRBS, it must use a very expensive phase modulator. Consequently, the production cost of a transmitter is increased.